Natural killer (NK) cells appear to be important participants in normal host responses to viral infection and in tumor surveillance because they lyse virally infected and tumor cells. However, the precise molecular mechanisms by which NK cells recognize their targets have not been elucidated. Recent evidence supports an inverse correlation between target cell MHC class I antigen expression and resistance to NK cell-mediated lysis. Our preliminary data strongly suggest that the murine NK cell surface antigen, Ly-49, interacts specifically with target cell MHC class I antigens. This interaction globally inhibits NK cell cytotoxic activity against target cells bearing specific MHC class I antigens, and results in the absence of the Ly-49+ NK cell subset in vivo. Therefore, we propose to: 1) Determine the specific MHC class I molecule that interacts with Ly-49. We will examine additional MHC congenic and recombinant, as well as Fl hybrid mice for absence of Ly-49 expression on NK cells and perform anti-MHC-specific mAb "blocking studies" in standard NK cell cytotoxicity assays. We will transfect targets with a specific MHC class I gene in an attempt to attempt specifically convert susceptible cells to targets that are resistant to Ly-49+ NK cell-mediated lysis. 2) Demonstrate directly that Ly-49 binds to MHC class I antigen. We propose to transfect Ly-49 into Ly-49- NK cells and directly show that Ly-49 itself is involved in the interaction with MHC class I. Using transfection and DNA amplification techniques, we will express Ly-49 at high levels on cell lines and examine whether these cells bind specifically to cells bearing the appropriate MHC class I antigen. We will produce a soluble, recombinant form of Ly-49 to measure the affinity and stoichiometry of the Ly-49-MHC interaction. 3) Determine the roles played by cations and carbohydrates in the interaction between Ly-49 (a member of the C-type lectin supergene family) and MHC class I in the cell-cell and soluble Ly-49 binding assays. 4) Examine the structural motifs in Ly-49 that are involved in binding to MHC class I by testing mutant forms of Ly-49 that are altered in the extracellular domain. To examine the possibility that Ly-49 mediates its effect by "negative signalling," we will transfect intracytoplasmic domain mutants of Ly-49 into Ly-49- NK cells. These studies should yield important new information regarding the molecular interactions between NK cells and their targets that regulate NK cell cytotoxic activity and that are responsible for MHC-related target cell resistance to NK cells. These studies will firmly establish a role for MHC antigens in this interaction and in shaping the NK cell repertoire. Finally, in view of the striking correlation between certain autoimmune diseases and MHC class I antigens, some of these diseases may reflect underlying NK cell dysfunction and the proposed studies may provide important clues to the pathogenesis of these disorders.